Fluid dispensers (or fluid applicators) such as drench guns or pour-on applicators are widely used in animal husbandry to deliver quantities of medicament or nutritional supplements to animals.
A typical drench gun consists of a chamber with a valve at either end. The inlet valve controls passage of fluid from a fluid reservoir to the drench gun, passing the liquid to be delivered into the barrel of the drench gun. When full or the desired amount of liquid is in the barrel, the operator activates a piston which forces the liquid from the barrel via an outlet valve.
A problem with this arrangement is that there can be inadvertent pressure applied to either the inlet or outlet valves which causes undesired leakage from the drench gun. The inlet and outlet valves are operable by fluid pressure. That is to say, passage of fluid past these valves requires an increase or decrease in fluid pressure which ideally is created by the user through actuation of the piston.
However, there can be occasions when the valves open due to undesired fluid pressure, resulting in fluid leakage.
Usually, this undesired fluid pressure causing the leakage is as a result of a “pressure pulse”, which is sometimes also referred to as “water hammer”. This phenomenon occurs when liquid supply is abruptly slowed or stopped as a result of the piston reaching its maximum or minimum displacement while the chamber is refilling.
The pressure pulse can cause one or both of the inlet and outlet valves to inadvertently open, allowing liquid to pass through the gun. This “pressure pulse” is also an issue in pour-on applicators and other fluid delivery devices.
Another source of undesired fluid pressure is when the drench gun is orientated in such a way that the reservoir supplying the liquid to the drench gun is raised above the drench gun in use.
Placing the reservoir higher than the drench gun is preferred, as gravity assists with the refilling of the drench gun. However, if the reservoir is too high, then the additional force of gravity acting on the fluid upstream of the inlet valve can cause the inlet valve to inadvertently open.
For example, when the reservoir is situated such that reservoir is higher than the drench gun outlet, there can be increased pressure on the valve of the drench gun when the drench gun is lowered or dropped by a user. For example, the user may be wearing a backpack reservoir.
Alternatively, if the reservoir is lower than the drench gun, then there is little risk of undesired pressure on the inlet valve, but then greater force is required to overcome gravity and refill the drench gun.
Although the leakage may be insignificant, it nonetheless is wasteful of the fluid being delivered. It also can mean that that an incorrect amount of liquid is being delivered to the animal. This can be a particular concern when medicaments are being delivered, as the animal may receive more than is desired.
The operator may also view the leakage as a fault with the drench gun, when in fact the drench gun is operating as it should.
Furthermore, an unexpected leakage can be messy with the fluid not being appropriately delivered to the animal, but elsewhere such as on the operator's clothes or on the ground.
To overcome this problem, it is common practice to use a relatively strong spring in the outlet valve. The valve is operable by fluid pressure acting on one side of the valve. When the pressure on the upstream side of the valve is greater than the downstream side, the valve opens.
It will be understood that the upstream side of the valve faces the direction of the fluid supply while the downstream side of the valve faces the nozzle.
The use of a spring to ensure the valve remains closed until fluid flow is desired, means that considerable force is required to be applied to the outlet valve in order overcome the spring sufficiently to allow liquid to pass. Thus, even though a pressure pulse may still occur, the force of the pulse is usually insufficient to cause the outlet valve open, thus the valve stays closed and prevents liquid from passing.
However, the use of a strong spring in the outlet valve means that a user must apply considerable force to operate the drench gun.
This can be a problem when the user needs to repeatedly deliver liquid in a short period of time, such as the case when dosing a herd of dairy cows. The user can quickly become fatigued when using the drench gun, and a repetitive strain injury can result.
This is especially the case if the fluid reservoir has been placed lower than the drench gun, as to overcome the effect of gravity on the fluid supply, a stronger handle spring is required, which must be biased by the user.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
Throughout this specification, the word “comprise”, or variations thereof such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications may be referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.